This invention relates to a hand wheel actuator for a steer-by-wire system of a vehicle.
Automobiles are conventionally equipped with a pair of front road wheels that are steered to enable the vehicle to turn left and right as it maneuvers on land. It is also known to provide for steering rear wheels in automobiles. In the past, vehicle steering systems commonly employed a mechanical linkage between the driver-operated hand wheel and the front road wheels of an automotive vehicle. As the driver rotated the hand wheel, a mechanical linkage through the vehicle""s tie-rods actuated the road wheels sometimes with the assistance of a power assist steering motor or hydraulic piston.
Recently, steer-by-wire steering systems have been introduced into automotive vehicles to provide road wheel steering function. Included in a typical steer-by-wire steering system is a hand wheel actuator for monitoring the angular position of the steering wheel, and road wheel motor actuators which are controlled by controllers in response to tracking the sensed angular displacement of the hand wheel from a central position. In contrast to prior steering systems, the steer-by-wire steering system does not employ a mechanical linkage between the steering wheel and the individual road wheels. Exemplary of such known steer-by-wire systems is commonly-assigned U.S. Pat. No. 6,176,341, issued Jan. 23, 2001 to Ansari, which is wholly incorporated herein by reference.
Prior art hand wheel actuators typically include a number of components including a hand wheel actuator, an electronic control unit, and a road wheel actuator. The hand wheel actuator includes sensors and motors for sensing the position of the hand wheel and/or torque transmitted through the steering shaft. The hand wheel actuator also includes an electric motor, which provides feedback to the driver and a agreeable steering feel. Thus, the hand wheel actuator and the electronic controller are in constant communication.
Previously, the electronic controller was housed in any convenient location behind the dashboard of the vehicle or in the engine compartment. However, this requires additional assembly steps.
Furthermore, as mentioned previously, hand wheel actuators typically include an electric motor to provide feedback to the driver and improve the feel of the hand wheel. Drivers expect hand wheels to have a certain feel that is consistent with a mechanical connection with the road wheels. However, absent such a mechanical connection, aspects of this desired feel must be synthesized. The required feel typically includes some inertia, some friction-related drag, and some centering force that biases the hand wheel toward a center position and a positive stop to limit rotation of the hand wheel to correspond to the limits of rotation of the road wheels. It would be desirable to provide such a mechanical steering feel enhancement device to provide as many aspects of the desired steering feel as possible in order to relieve the electric motor of these tasks. However, prior art mechanical steering feel enhancement devices required extra assembly steps and were therefore cumbersome to install. In addition, prior art return-to-center springs have had the draw back that the force is linearly proportional to displacement. It would be desirable to provide a mechanical steering feel enhancement device that is simple to install or assemble, and provides a satisfactory steering feel.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a hand wheel actuator for a steer-by-wire steering system having a steering feel that meets drivers"" expectations is provided with a first housing having an aperture, a steering shaft extending through the aperture and rotatably supported by the housing, an electric motor, a position sensor and torque sensor, and a mechanical steering feel enhancement device. The position and torque sensors are positioned within said first housing and configured to detect the angular displacement of the steering shaft and torque applied against the steering shaft respectively. The mechanical steering feel enhancement device is attached to the first housing and provides at least one of a positive stop function, a return-to-center function, or a friction/weight function by mechanical means. The mechanical steering feel enhancement device is in mechanical communication with said steering shaft and is disposed in a second housing and existing as a separate unitary subassembly prior to being installed on said first housing. The electric motor is mounted to one of the first housing and second housing and has an output shaft that is in mechanical communication with the steering shaft for imparting a torque to the steering shaft.